In general, the absorption capacity of an absorbent article is designed according to the intended use. Disposable diapers, for example, are used for a wide range of wearers from newborns to adults who need care and should be designed to have varied absorption capacities according to the wearer.
For instance, disposable diapers for newborns, who usually have loose and runny stools, should be designed to have enhanced stool absorbency in the portion from the longitudinal middle to the rear (back side). Some of those for infants or adults are designed to have increased stool absorption performance in the portion from the longitudinal middle to the front (stomach side) so as to absorb a large quantity of urine. The former diapers (for newborns) may hardly leak even without a high total absorption capacity. On the other hand, the latter diapers (for infants or adults), which are chiefly contemplated for absorbing a large quantity of urine, must have an increased absorption capacity in the portion from the longitudinal middle to the front. Known techniques for meeting this demand include (i) fabricating an absorbent member having a polymer-rich front portion by patternwise scattering superabsorbent polymer particles and (ii) fabricating an absorbent member having a two-ply structure in the front portion (e.g., JP-A-8-66426).
The method involving patternwise scattering superabsorbent polymer particles requires not only a special patternwise scattering unit but a unit for precisely positioning a scattering pattern on sheeting on which the polymer is scattered. Additionally, it is difficult to monitor the position of the scattering pattern on the production line. Sometimes there are cases in which the polymer particles fall off from the polymer-rich portion, or the polymer particles can cause gel blocking while in use, failing to exhibit absorption capability as expected.
An absorbent member having a two-ply structure only in the front portion thereof includes (a) one composed of a lower absorbent layer and a separately prepared upper absorbent layer laid on part of the lower absorbent layer and (b) one prepared by turning up a single-layered absorbent member to make a two-plied portion. Both absorbent members (a) and (b) require complicated equipment for production and are therefore costly for performance. That is, production of the former (a) involves fabricating two different absorbent layers and therefore needs as many apparatus (e.g., air-laying apparatus) as the absorbent layers. Production of the latter (b) requires an extract folding machine, etc. Moreover, the polymer of the upper ply can cause gel blocking upon absorption and swelling, which may hinder smooth migration of the absorbed liquid to the lower ply. It can follow that the absorbent member fails to manifest the absorbing performance in parallel to the amount of the absorbing materials, such as a polymer and pulp fiber. Gel blocking can also cause leaks. Furthermore, these absorbent members with a two-ply structure only in the front portion unavoidably have a difference in stiffness between the two-plied front portion and the single-plied rear portion, which can give a wearer discomfort.
JP-A-62-206071 discloses a technique for producing an absorbent pad having a higher basis weight in a certain zone than other zones, in which a web-forming plate on which a fibrous material is air-laid has a specifically contoured recess, and the space under the recess is designed so as to apply a high negative pressure to the recess. The publication refers only to deposition of a fibrous material, being silent on a mixed system of a superabsorbent polymer and pulp. Furthermore, because the web-forming plate has, in the zone where fibers are to be deposited with a higher basis weight, a recess surrounded by tapered walls, the suction force is not directly exerted to this zone. It is very likely that the resulting fiber-laid structure has physical discontinuity between the portion formed in the recess and the portion formed on the flat zone of the web-forming plate.
JP-A-2000-234255 discloses a method of producing an air-laid fiber article having regions which are different in height and density which comprises accumulating a raw material by suction in a recess having openings, wherein the recess has its bottom divided into a plurality of suction regions having different open area ratios. The publication has no mention of production using a mixed system of a superabsorbent polymer and pulp fiber. There is a description about the technique for providing the bottom of the recess with a level difference so as to control the height and density, but there is no description about controlling the basis weight. Besides, there is a possibility that the absorbent member thus produced becomes out of shape because of this level difference while the absorbent member is conveyed.
Absorbent members comprising a cellulosic fiber, such as pulp fiber, and a polyacrylic acid-based absorbent polymer have been widely used in absorbent articles for absorbing and retaining liquid body waste, such as disposable diapers, sanitary napkins, and panty liners. Absorbent members of the same composition have come to be employed for other applications.
In the development and quality evaluation of these kinds of products, it is beneficial to accurately determine the amount of the fiber and/or the absorbent polymer making up the absorbent member. However, conventional methods of quantitative determination have such disadvantages as low accuracy and a complicated operation.
It is possible, for example, that an absorbent member made of an absorbent polymer and pulp fiber is disintegrated and separated by screening through a sieve of prescribed mesh size into the absorbent polymer and the pulp fiber. However, it is difficult to choose a sieve of proper mesh size unless the particle size of the absorbent polymer and the fiber length of the pulp fiber are known. Even though a proper sieve is selected, time-consuming and laborious procedures, such as condition setting, have to be taken so as to minimize scatter of measured values, and measurement must be repeated for accuracy.